Sensory, motor, and pain thresholds for stimulation with medium frequency alternating current

To investigate the effect of frequency of alternating current on the sensory, motor, and pain thresholds in normal subjects, and to establish the optimal frequency for motor stimulation with minimal subject discomfort.

A repeated measures design using two groups of 11 subjects.

A laboratory setting was used.

Participants were volunteers who met the inclusion criteria.

Alternating current with 20 different carrier frequencies between 1 and 35 kHz, all modulated at 50 Hz, was applied to each subject on two separate occasions. For half the subjects, the frequency was increased or decreased sequentially (reversed on second occasion), and for the other half, it was applied in a different random order on each occasion.

The voltage at the sensory threshold was recorded for each applied frequency. This was subsequently repeated for motor and pain thresholds.

Sensory, motor, and pain thresholds decreased with increasing frequency between 1 kHz and approximately 10 kHz. Above 10 kHz, the thresholds increased. The ratio pain threshold/sensory threshold increased systematically with increasing frequency over the range examined. By contrast, the ratio pain threshold/motor threshold showed a distinct maximum at a frequency of 10 kHz. Marked accommodation to motor and pain fiber stimulation was observed.

For comfortable sensory stimulation, a high frequency of alternating current is preferable. Discrimination between pain and motor stimulation is maximal at a frequency of approximately 10 kHz. This suggests that the optimal frequency for comfortable motor stimulation, one that is least likely to elicit pain fiber stimulation, is close to 10 kHz.